Most of the heavy metals have long been known for their significant bactericidal effects, especially silver. There are references in ancient literature to the use of silver lined vessels being used to store drinking water because of the disease-free experiences of those who did. Physicians through the years since the days of Christ have used silver in treating human ailments, and modern science started recording work done on ascertaining silver's beneficial properties in disinfection in the 1800's. Silver's widespread use as a water disinfectant was initiated early in the 1900's and especially in the late 1920's and early 1930's. There exists much evidence giving support to the concept of using silver for its bactericidal effects. Further, its use in the treatment of drinking water supports the conclusion that no hazard to human beings will result from its use in concentrations much larger than necessary for bacteria control. There is today increasing understanding of the toxicological properties from chloridated hydrocarbons remaining in water following the application of chlorine, and of the ever-present irritations to human beings caused by a combination pH/chlorine phenomena. The need for a superior alternate to chlorine as a disinfectant in water is very significant.
When silver is used as a water disinfectant, the actual bacteria-killing agent is the silver ion. Silver metal releases this ion very slowly and thus a very large surface area is needed to impart a significant quantity of silver to anything approaching the volumes of water found in large bodies such as swimming pools. Some prior art processes using silver for disinfection of water employ very large surface area media, such as carbon or synthetic resins coated with silver, and arranged in a column or tube through which water to be treated must pass.
It is believed, however, that coating the granules of fine sand in a sand filter, or coating many of the diatom skeletons in a diatomaceous earth is a far superior way of achieving the surface area requirement. Coating of sand or diatomaceous earth with silver to produce a sterilizing filter for water supplies, swimming pools and the like is known in the prior art. It is also known that the impregnation of such material with silver should be carried out under conditions which produce a uniform distribution of the impregnating material. An example of this teaching is set forth in U.S. Pat. No. 1,473,331 to Bechhold. Bechhold describes a filter material such as diatomaceous earth which may be impregnated with silver by soaking in a solution of silver nitrate, after which the product is dried and treated in various ways in order to fix the resultant silver coating. The patent recognizes that the impregnation should be carried out under conditions which insure uniform distribution of the impregnating material and suggests the use of heat for this purpose. However, the patent stops short of any discussion of the desired temperature environment other than to imply that it should be sufficient to reduce a metallic compound coating into elemental form. Other patents considered by Applicants in this regard include the following: U.S. Pat. Nos.: 1,557,234; 1,685,204; 2,167,225, 3,257,315; 3,168,444; 3,372,808; Japanese Patent Application No. 14338/1977, filed Dec. 1, 1977, published on June 26, 1979.
It is well known that to disinfect a given unit weight of water with a silver coated filter material, it is not the thickness of the silver layer itself which is essential but the total of the silver coated surfaces of all the particles of the material in contact with the water. It is applicants' conviction that in the prior art, despite its familiarity with the use of silver as a sterilizing filter agent, and in particular the use of this metal to coat diatomaceous earth filters, insufficient attention has been directed to maximizing the efficient deposition of elemental silver on the individual particle surfaces of such filters, or to analyzing the mechanism whereby this occurs.
It is therefore, a general object of this invention to provide an improved sterilizing filter material and process for making the same.
It is a more particular object of this invention to provide an improved filter composed of sand or diatomaceous earth which enables the coating thereon of a substantially uniform layer of elemental silver to enhance the effectiveness of transmission of such silver to a surrounding body of water.